Making a texture partially see-through
One way you could do this is to dynamically create a mask from your marble texture that is used as a mix factor between high translucence and low/zero translucence areas of the material.  So the veins would be low translucence, the whitish parts higher translucence.



This node setup shows the concept in action.  At the bottom on the left is the marble texture I was working with; on the right is a preview of the mask, created using a color ramp.



They say you should always "add" translucence to your other shaders, but honestly I didn't like the way that turned out . . . too bright, hard to see the specularity.  So I used a mix shader instead.  Maybe less physically correct, but I think it looks better.